This invention relates to a magnetic recording medium for use with a magnetic disk apparatus for carrying out information recording and reproducing operations.
In order to achieve an increase in recording density of a HDD (hard disk drive), a decrease in medium noise is indispensable. In the past, the decrease in medium noise is achieved by improving a film structure, a film material of a magnetic recording medium, or reduction of a product Mr•t of the magnetic disk, with the help of uninterrupted improvement in output characteristics of a magnetic head. The product Mr•t is a product of residual magnetization (Mr) of a magnetic layer of the magnetic disk and a film thickness (t) of the magnetic layer of the magnetic disk.
The reduction in Mr•t is extremely effective in improving R/W (read/write) characteristics but simultaneously causes a problem of a thermal decay characteristic. The decrease in Mr•t, i.e., the decrease in film thickness of a magnetic layer brings about miniaturization of the grain size of the magnetic layer, resulting in reduction of the medium noise. However, miniaturized crystal grains no longer have a coercive force (Hc) sufficient to hold recorded magnetization as a recorded signal. This results in a phenomenon that the recorded signal is attenuated. This phenomenon is called thermal decay.
In order to prevent the phenomenon (thermal decay) that the recorded signal is attenuated, various film structures have been proposed. Attention is recently attracted to one of the film structures which is called an AFC (Anti-Ferro-Coupled-film) structure (see Japanese Unexamined Patent Publication JP 2001-56923 A).
Japanese Unexamined Patent Publication JP 2001-56923 A proposes a magnetic recording medium in which a magnetic recording layer formed on a substrate has a multilayer structure. Specifically, the magnetic recording layer is divided by a nonmagnetic separation layer (Ru, Rh, Ir, or the like) into upper and lower magnetic layers. The upper and the lower magnetic recording layers divided by the nonmagnetic separation layer have magnetizing directions anti-parallel to each other. With this structure, the magnetic recording medium is excellent in thermal decay characteristic.
However, a film using the AFC structure is inevitably increased in magnetic layer thickness in total although the thermal decay characteristic is excellent. The increase in magnetic layer thickness results in a decrease in coercive force squareness ratio (S*). The increase in magnetic layer thickness also results in an increase in grain size of the magnetic layer. In this event, the medium noise (S/N ratio) is deteriorated. In addition, the output (LF (Low Frequency)) and the pulse width (PW) are deteriorated also. Thus, such recording/reproducing characteristics may fail to meet future demands for a more and more increase in recording density.